Ecosystem allometry: the scaling of nutrient stocks and primary productivity across plant communities

A principal challenge in ecology is to integrate physiological function (e.g. photosynthesis) across a collection of individuals (e.g. plants of different species) to understand the functioning of the entire ensemble (e.g. primary productivity). The control that organism size exerts over physiological and ecological function suggests that allometry could be a powerful tool for scaling ecological processes across levels of organization. Here we use individual plant allometries to predict how nutrient content and productivity scale with total plant biomass (phytomass) in whole plant communities. As predicted by our model, net primary productivity as well as whole community nitrogen and phosphorus content all scale allometrically with phytomass across diverse plant communities, from tropical forest to arctic tundra. Importantly, productivity data deviate quantitatively from the theoretically derived prediction, and nutrient productivity (production per unit nutrient) of terrestrial plant communities decreases systematically with increasing total phytomass. These results are consistent with the existence of pronounced competitive size hierarchies. The previously undocumented generality of these 'ecosystem allometries' and their basis in the structure and function of individual plants will likely provide a useful quantitative framework for research linking plant traits to ecosystem processes.     

Decoupling the direct and indirect effects of nitrogen deposition on ecosystem function

Elevated nitrogen (N) inputs into terrestrial ecosystems are causing major changes to the composition and functioning of ecosystems. Understanding these changes is challenging because there are complex interactions between 'direct' effects of N on plant physiology and soil biogeochemistry, and 'indirect' effects caused by changes in plant species composition. By planting high N and low N plant community compositions into high and low N deposition model terrestrial ecosystems we experimentally decoupled direct and indirect effects and quantified their contribution to changes in carbon, N and water cycling. Our results show that direct effects on plant growth dominate ecosystem response to N deposition, although long-term carbon storage is reduced under high N plant-species composition. These findings suggest that direct effects of N deposition on ecosystem function could be relatively strong in comparison with the indirect effects of plant community change.     

Simulating root responses to grazing of a Mongolian grassland ecosystem

A new Sim-CYCLE grazing model has been obtained by combining a grazing model (Seligman et al. 1992, Ecol. Model. 60: 45–61) with the Sim-CYCLE model (Ito and Oikawa 2002, Ecol. Model. 151: 143–176). The new model has been validated against a set of field data obtained at Kherlen Bayaan-Ulaan (KBU) grassland. On the basis of the model, the root responses to grazing of KBU grassland have been studied under different conditions of stocking rates and precipitation. Model results indicate that both below-ground biomass (BB) and below-ground net primary production (BNPP) generally decrease with increasing stocking rate. However, if stocking rate is not higher than 0.7 sheep ha⁻¹, a sustainable state of the grassland ecosystem can be achieved after about 100 years, which suggests that the maximum sustainable stocking rate at KBU should be 0.7 sheep ha⁻¹. At the sustainable state, the maximum BB in a year is about 11 Mg DM ha⁻¹ under non-grazing condition, 5 Mg DM ha⁻¹ under 0.4 sheep ha⁻¹ stocking rate, and 4 Mg DM ha⁻¹ under 0.7 sheep ha⁻¹ stocking rate; the BNPP is 1.3 Mg DM ha⁻¹ year⁻¹ under non-grazing condition, and 0.6 Mg DM ha⁻¹ year⁻¹ under 0.4 sheep ha⁻¹ stocking rate, and 0.4 Mg DM ha⁻¹ year⁻¹ under 0.7 sheep ha⁻¹ stocking rate. Ratio of non-assimilation organ to assimilation organ (C/F) increases with increasing stocking rate. The C/F ratio is 10.99 under non-grazing conditions, and 12.11 under 0.7 sheep ha⁻¹ stocking rate. Root turnover rate decreases with increasing stocking rate. The rate is 12% each year under non-grazing conditions, and 11% each year under 0.7 sheep ha⁻¹ stocking rate. In addition, the effect of grazing on the grassland ecosystem under different scenarios of precipitation is also analyzed. Both BB and BNPP increase with increased precipitation, and vice versa. When precipitation is set to be 10% higher than the averaged from 1993 to 2002, the maximum sustainable stocking rate is 0.8 sheep ha⁻¹, and when the precipitation is set to be 15% lower than the averaged, the maximum sustainable stocking rate is 0.6 sheep ha⁻¹.     

Carbon balance of a European mountain bog at contrasting stages of regeneration

Carbon dioxide and methane (CH4) fluxes were measured in a cutover bog of the Jura Mountains (France) together with biotic and abiotic variables for two entire vegetation periods in order to compare the carbon balance of the bog at three stages of regeneration. Among all factors, air temperature and vegetation index (including leaf area of vascular plants, bryophyte density and bryophyte desiccation) were the two main determinants of ecosystem respiration and gross photosynthesis at light saturation. During 2004 and 2005, the vegetated plots acted as carbon sinks. Net carbon exchange ranged between 67 and 166 g C m(-2) yr(-1) for the Eriophorum-dominated plots and between 93 and 183 g C m(-2) yr(-1) for the Sphagnum-dominated plots. The bare peat plots represented a net carbon source (between -19 and -32 g C m(-2) yr(-1)). Methane fluxes accounted for a very small part of the total carbon efflux (< 2%). The recovery of vegetation in our naturally regenerating bog was beneficial for the carbon sequestration after the relatively short period of 20 yr.     

Atmospheric impact of bioenergy based on perennial crop (reed canary grass,Phalaris arundinaceae,L.) cultivation on a drained boreal organic soil

Marginal organic soils, abundant in the boreal region, are being increasingly used for bioenergy crop cultivation. Using long‐term field experimental data on greenhouse gas (GHG) balance from a perennial bioenergy crop [reed canary grass (RCG), Phalaris arundinaceae L.] cultivated on a drained organic soil as an example, we show here for the first time that, with a proper cultivation and land‐use practice, environmentally sound bioenergy production is possible on these problematic soil types. We performed a life cycle assessment (LCA) for RCG on this organic soil. We found that, on an average, this system produces 40% less CO₂‐equivalents per MWh of energy in comparison with a conventional energy source such as coal. Climatic conditions regulating the RCG carbon exchange processes have a high impact on the benefits from this bioenergy production system. Under appropriate hydrological conditions, this system can even be carbon‐negative. An LCA sensitivity analysis revealed that net ecosystem CO₂ exchange and crop yield are the major LCA components, while non‐CO₂ GHG emissions and costs associated with crop production are the minor ones. Net bioenergy GHG emissions resulting from restricted net CO₂ uptake and low crop yields, due to climatic and moisture stress during dry years, were comparable with coal emissions. However, net bioenergy emissions during wet years with high net uptake and crop yield were only a third of the coal emissions. As long‐term experimental data on GHG balance of bioenergy production are scarce, scientific data stemming from field experiments are needed in shaping renewable energy source policies.     

The systems approach to the prespore-specific activation of sigma factor SigF in Bacillus subtilis

The prespore-specific activation of sigma factor SigF (σ^F) in Bacillus subtilis has been explained mainly by two factors, i.e., the transient genetic asymmetry and the volume difference between the mother cell and the prespore. Here, we systematically surveyed the effect of these two factors on sporulation using a quantitative modeling and simulation architecture named hybrid functional Petri net with extension (HFPNe). Considering the fact that the transient genetic asymmetry and the volume difference in sporulation of B. subtilis finally bring about the concentration difference in two proteins SpoIIAB (AB) and SpoIIAA (AA) between the mother cell and the prespore, we have surveyed the effect of AB and AA concentration on the prespore-specific activation of σ^F occurring in the early stage of sporulation. Our results show that the prespore-specific activation of σ^F could be governed by the ratio of AA to AB rather than their concentrations themselves. Our model also suggests that B. subtilis could maximize the ratio of AA to AB in the prespore and minimize it in the mother cell by employing both the transient genetic asymmetry and the volume difference simultaneously. This might give a good explanation to the co-occurrence of the transient asymmetry and the volume difference during sporulation of B. subtilis. In addition, we suggest for the first time that the σ^F activation in the prespore might be switched off by the decrease in the ratio of AA to AB after the transient genetic asymmetry is to an end by completion of DNA translocation into the prespore.     

Interspecific correlates of plasticity in relative growth rate following a decrease in nitrogen availability

Background and Aims

Nitrogen availability varies greatly over short time scales. This requires that a well-adapted plant modify its phenotype by an appropriate amount and at a certain speed in order to maximize growth and fitness. To determine how plastic ontogenetic changes in each trait interact and whether or not these changes are likely to maximize growth, ontogenetic changes in relative growth rate (RGR), net assimilation rate (NAR), specific leaf area (SLA) and root weight ratio (RWR), before and after a decrease in nitrogen supply, were studied in 14 herbaceous species.

Methods

Forty-four plants of each species were grown in hydroponic culture under controlled conditions in a control treatment where the supply of nitrogen remained constant at 1 mm, and in a stress treatment where the nitrogen supply was abruptly decreased from 1 to 0·01 mm during the growth period.

Key Results and Conclusions

In the treatment series, and in comparison with the control, NAR and RGR decreased, RWR increased, and SLA did not change except for the timing of ontogenetic change. Species having greater increases in the maximum rate of change in RWR also had smaller reductions in RGR; plasticity in RWR is therefore adaptive. In contrast, species which showed a greater decrease in NAR showed stronger reductions in RGR; plasticity in NAR is therefore not adaptive. Plasticity in RGR was not related to plasticity in SLA. There were no significant relationships among the plasticities in NAR, RWR or SLA. Potentially fast-growing species experienced larger reductions in RGR following the nitrogen reduction. These results suggest that competitive responses to interspecific competition for nitrogen might be positively correlated with the plasticity in the maximum rate of change in RWR in response to a reduction in nitrogen supply.     

陕北黄土高原植被净初级生产力的估算

基于MODIS和地面气象数据,利用改进的CASA模型,模拟分析了2005年陕北黄土高原地区的植被净初级生产力(NPP)及其时空分布.结果表明:1)根据生态生理过程模型针对不同土地覆被类型选择不同的月平均最大光能利用率,比传统CASA模型中使用固定的全球月平均最大光能利用率进行NPP估算,更符合陕北黄土高原地区的实际情况;在估算植被参数时引入植被覆盖分类,以及利用陕北黄土高原2005年时序NDVI进行土地覆被分类的同时,结合1:100万中国植被图和实地调查情况对分类结果进行修正,可提高分类的精度,从而提高模型估算的精度.2)通过不同模型之间和与陕北部分地区实际调查数据进行比较,显示改进后的CASA模型对区域陆地植被NPP的模拟效果较好,可应用于陕北黄土高原乃至周边地区NPP的计算中.3)2005年陕北黄土高原植被净第一性生产量估计值为4.76×10~(13) g C,约占全国总NPP的1.5%,植被平均NPP为447.3 g C·m~(-2)·a~(-1),高于1992-2000年全国陆地NPP平均值323.8 g C·m~(-2)·a~(-1).4)在NPP的空间分布上,总体趋势是由东南向西北递减,其中最高值出现在东南部的黄龙山次生林区(1087g C·m~(-2)·a~(-1));西北部的荒漠植被覆盖度极低,平均NPP仅为205.0 g C·m~(-2)·a~(-1).5)陕北黄土高原NPP的季节变化明显,其中4月中旬至10月中旬6个月生长季时间里的NPP可占到全年的91.5%,而7月中旬至8月中旬间该区的净初级生产力达到年内的极大值,可占全年的37.8%.

Abstract：

Based on the data from MODIS (Moderate-resolution Imaging Speetroradiometer) and meteorological observatories, and by using improved CASA (Carnegie-Ames-Stanford Approach) model, the vegetation net primary productivity (NPP) and its spatiotemporal distribution on the North Shaanxi Loess Plateau in 2005 were simulated and analyzed. Comparing with the traditional CASA model which only uses a universal mean annual maximum light use efficiency (LUE), the estimated regional NPP by the improved CASA model was more precise, because this improved model used the LUE parameters of different vegetation covers. The detailed land cover classifica-tion also contributed to the increase of the precision via introducing the time-series Normalized Different Vegetation Index (NDVI) and ground survey data to modify and adjust the original clas-sification system based on vegetation map (1: 1000000). The testing of the simulation results from different models with the ground survey data in North Shaanxi showed that the estimation by the modified CASA model was much closer to the real survey data, implying the potential practi-cal significance of this model in estimating the vegetation NPP in North Shaanxi Loess Plateau and its adjacent areas. In 2005, the NPP in North Shaanxi was estimated as 4. 76×1013 g C, ac-counting for about 1.5% of China' s terrestrial total NPP, and the mean NPP was 447.3 g C·m~(-2)·a~(-1), being much higher than that of China' s terrestrial vegetation (323.8 g C·m~(-2)·a~(-1)) in 1982-2000. The spatial distribution pattern of the vegetation NPP showed an apparently declining trend from the southeast to the northwest, with the highest value of 1087 g C·m~(-2)a~(-1) occurred in the broadleaved-and conifer-mixed forests of Huanglong Mountain in southeast part of the region. The mean NPP of desert vegetation in the whole region was the lowest, only about 205.0 g C·m~(-2) ·a~(-1). An obvious seasonal variation of the NPP was observed. The NPP in growth season (from April to October) took about 91.5% of the total in the year, and the peak occurred in mid-July to mid-August, amounting to 37.8% of the total.     

Do species evenness and plant density influence the magnitude of selection and complementarity effects in annual plant species mixtures?

Abstract Plant species richness influences primary productivity via mechanisms that (1) favour species with particular traits (selection effect) and (2) promote niche differentiation between species (complementarity). Influences of species evenness, plant density and other properties of plant communities on productivity are poorly defined, but may depend on whether selection or complementarity prevails in species mixtures. We predicted that selection effects are insensitive to species evenness but increase with plant density, and that the converse is true for complementarity. To test predictions, we grew three species of annuals in monocultures and in three‐species mixtures in which evenness of established plants was varied at each of three plant densities in a cultivated field in Texas, USA. Above‐ground biomass was smaller in mixtures than expected from monocultures because of negative ‘complementarity’ and a negative selection effect. Neither selection nor complementarity varied with species evenness, but selection effects increased at the greatest plant density as predicted.     

Effects of Simulated Acid Precipitation on Photosynthesis,Chlorophyll Fluorescence,and Antioxidative Enzymes in Cucumis sativus L.

The effects of simulated acid rain on gas exchange, chlorophyll fluorescence, and anti-oxidative enzyme activity in cucumber seedlings (Cucumis sativus L. cv. Jingchun No. 4) were investigated. Acid rain significantly reduced net photosynthetic rate and mainly non-stomatal factors contributed to the decrease of photosynthesis during the experimental period. The reduced photosynthesis was associated with a decreased maximal photochemical efficiency (F_v/F_m) and the average quantum yield of the photosystem 2 (PS2) reaction centres (_PS2). Meanwhile, acid rain significantly increased the activities of guaiacol peroxidase (GPX) and superoxide dismutase (SOD), but decreased the activity of catalase (CAT) together with an increased content of malonyldialdehyde (MDA), Hence the changes in photosynthesis in acid rain treatment might be a secondary effect of acidity damage probably due to lipid peroxidation of lipids and proteins in thylakoid membrane rather than direct effect on PS2 reaction centre.